(a) Field of the Invention
The present invention relates to a magnetoresistive element, and to a magnetoresistive sensor using such an element.
(b) Description of the Related Art
A so called magnetoresistive (MR) sensor or MR head has been known in the form of magnetic read sensor, which can read out data from a magnetic layer at a large linear density. The electric resistance of an MR sensor varies as a function of the strength and direction of magnetic fluxes sensed by a read element, whereby a magnetic field signal corresponding to the resistance is output from the MR sensor. In such a conventional MR sensor, the read element operates based on the anisotropic magnetoresistance (AMR) effect in which the resistance of the read element varies in proportion to the square of the cosine of the angle between the magnetization and the direction of sense current flowing through the element. The AMR effect is described in detail in "Memory, Storage, and Related Applications," by D. A. Thompson, et al., IEEE Trans. Mag. MAG-11, p. 1039 (1975).
Another paper has reported a more remarkable magnetoresistance effect in which the resistance of a layered magnetic sensor varies due to both spin-depending transfer of conduction electrons between magnetic layers via a non-magnetic layer, and spin-depending scattering at the interfaces between the layers accompanying the transfer of conduction electrons. This magnetoresistance effect is variously referred to as "giant magnetoresistance effect" or "spin valve (SV) effect." Such an MR sensor fabricated of the appropriate materials provides improved sensitivity and greater change in resistance than observed in sensors utilizing the AMR effect. In this type of MR sensor, the in-plane resistance between a pair of ferromagnetic layers, separated by a non-magnetic layer, varies in proportion to the cosine of the angle between the magnetization in the two ferromagnetic layers.
Patent Publication No. JP-A-90-61572 (corresponding to U.S. Pat. No. 4,949,039) discloses a layered magnetic structure which yields enhanced MR effect caused by anti-parallel alignment of the magnetizations in the magnetic layers. In this patent publication, ferromagnetic transition metals and alloys are listed as possible materials for use in the layered structure. The patent publication also discloses a structure having an antiferromagnetic layer added to at least one of two ferromagnetic layers separated by an intermediate layer, as well as suitability of FeMn for use as the antiferromagnetic layer.
Patent Publication No. JP-A-92-358310 (priority claimed from a U.S. application filed on Dec. 11, 1990) discloses an MR sensor having two thin layers of a ferromagnetic material separated by a thin film layer of a non-magnetic metallic material. When no magnetic field is applied to the sensor, the magnetizations of the two ferromagnetic thin film layers intersect each other substantially at right angles. When a magnetic field is applied to the sensor, the resistance between the two magnetic layers varies in proportion to the cosine of the angle between the magnetizations of the two ferromagnetic layers, independent of the direction of the current flowing through the sensor.
Patent Publication No. JP-A-94-203340 (corresponding to U.S. Pat. No. 5,301,079) discloses an MR sensor which has two thin film layers of a ferromagnetic material separated by a thin film layer of a non-magnetic metallic material and which operates based on the effect similar to that of the sensor disclosed in the above patent publication. However, in this MR sensor, when no magnetic field is applied to the sensor, the direction of magnetization in an adjacent antiferromagnetic layer adjacent to one of the two ferromagnetic thin film layers is maintained perpendicular to the others of ferromagnetic layers.
When a magnetoresistive element utilizing the SV effect is used as a magnetoresistive sensor, it is necessary to optimize the operational point (or cross point) for zero applied magnetic field, as in conventional magnetoresistive sensors utilizing the AMR effect. In the magnetoresistive element utilizing the SV effect, the shape of the element affects a playback output from a head. Moreover, in an SV element utilizing a non-conductive material as an antiferromagnetic material, the thickness of the antiferromagnetic material affects the gap length of a shielded magnetoresistive element, and also affects the waveform of a signal reproduced by the shielded magnetoresistive head.